1. Field of the Invention
The invention relates to test and measurement apparatus for semiconductor wafer processing system and, more particularly, to a composite diagnostic wafer for measuring both ion energy and ion current at various locations on the diagnostic wafer during plasma generation by a semiconductor wafer processing system.
2. Description of the Prior Art
Ion current flux and ion energy are important parameters of a plasma contained by a reaction chamber within a semiconductor processing system. These parameters define the effectiveness of the semiconductor wafer processing system in processing a wafer. Specifically, the ion current flux affects the uniformity of the etch process and indicates potential damage to a wafer. Additionally, ion energy affects selectivity of the etch, etch rate uniformity and residue control. Since these parameters are so important to the etch process, the measurement of both ion current and ion energy at a given location within the chamber is important to characterizing the effectiveness of the plasma in processing a wafer.
Typically, to measure the ion current, an ion current probe, similar to a Langmuir probe, is used. To measure the distribution of current at the surface of a wafer, one or more current probes are affixed to one surface of a placebo wafer, i.e., an anodized aluminum disk having the same size and shape as a semiconductor wafer. The placebo wafer is then positioned within a semiconductor processing system in a similar location as a semiconductor wafer would typically be located. Once a plasma is generated by the processing system, the ion current probes are biased negatively to collect ions from the plasma. Consequently, an electric current is produced in a wire attaching the probe to a current meter. The measured current is indicative of the number of ions incident upon the current probe at that location on the placebo wafer. By judiciously positioning the current probes in an array on the surface of the placebo wafer, the ion currents measured at each individual current probe are combined to estimate the ion current distribution over the surface of the placebo wafer. This current distribution is indicative of the ion current flux within the plasma.
Separately, an ion energy analyzer is generally imbedded into a support structure for the semiconductor wafer, e.g., such support structures are known as wafer chucks, susceptors, or wafer pedestals. An ion energy analyzer is a well known device for determining the energy properties of ions within a plasma. For a detailed description of an ion energy analyzer, see R. L. Stenzel et al., "Novel Directional Ion Energy Analyzer", Rev. Sci. Instrum. 53(7), July 1982, pp. 1027-1031 which is hereby incorporated by reference. As described therein, a traditional ion energy analyzer contains a metallic collector, a control grid, and a floating grid, all formed into a cylindrical stack where the collector and each grid are separated by a ceramic insulating washer. Specifically, the collector is a negatively biased metallic disk. The negative bias repels electrons from the collector and attracts ions to the collector. The control grid is positively biased such that ions with energies that do not exceed the positive bias are rejected by the analyzer. As such, the control grid is used to select ions for collection that have energy levels greater than a specified energy level and reject all others. The unbiased (floating) grid is either a mesh screen or a micro-channel plate, being unbiased, simulates the surface of a semiconductor wafer.
In operation, the ion energy analyzer imbedded in the pedestal is either used to measure the ion energy prior to having a wafer placed upon the pedestal or a specially designed wafer, having a hole to expose the energy analyzer to the plasma, is placed upon the pedestal. Once the plasma is established in the chamber, ions having energies exceeding the control grid bias are collected by the collector plate and create an electrical current in an ammeter connected to the collector plate. The energy of the ions in the plasma is determined by adjusting the control grid bias and monitoring the current measured by the ammeter.
The prior art teaches separately using current probes on a placebo wafer and an ion energy analyzer imbedded in a wafer pedestal. As such, to determine both ion current and ion energy, first the ion current is measured and then the ion energy is measured, or vice versa. Due to the necessity to conduct two sequential measurements, the duration of plasma test is much longer than each of the measurements taken singly. Furthermore, since the current and energy measurements are separated in time, such measurements can be inaccurate.
Therefore, a need exists in the art for a single diagnostic wafer combining both one or more ion current probes with one or more ion energy analyzers to facilitate substantially simultaneous ion current and energy measurement within a semiconductor wafer processing system.